Great manager, but I honestly don't think he'll dominate in the Premier League/Champions League like some think. By no means was it easy to to win the treble with Barca and take them to a level that very few managers have ever taken a team before, but they already had the foundation, and 5/6 players who were at the peak of their players.
Same can be said about Bayern, who had won the treble prior to Pep joining. Again, he's done very well at Bayern, but apart from Dortmund, who he stole two of their best players off and made them increasingly weaker, he didn't really have any one to rival them in the league. I guess his toughest test come for him in the CL, where he failed miserably against a very good Barca side.
Be a lot different at City, in my opinion. For starters, he won't be walking into a team who has the same level of players he had at Barcelona and Bayern. Yes, City arguably have the best squad in the Premier League, but they still have many holes in their current starting XI, in my opinion, and are quite a way off competing with the likes of Bayern, Barca, Madrid, etc. This is not to say the can't reach their level, but I don't think it'll be something that can be rectified in one season.
They will also have a lot more teams to compete with. I can imagine, City, United, Chelsea, Arsenal, Liverpool and Spurs will all want to be competing for the title next season, which is a lot different to what he's been used to, where, more often than not, it's been 2 or 3 teams.
All in all, I think Pep will do good. He'll bring his usual expansive, possession football to the league. Bit like Van Gaal, but with a lot more attacking impetus and risk in his play. I just want to know if, in a year or so, Enrique will be rated the same as Pep? Yes, Pep created that superb Barca team and blew teams away, but Enrique has more or less a completely different side and is doing just as well, winning the treble in his first year and still on for it this year.